The present invention relates generally to multilayer imaging media and, more particularly, to laminated photothermographic films.
Silver-containing photothermographic imaging materials that are developed with heat and without liquid development have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation (for example, visible, ultraviolet or infrared radiation) and developed by the use of thermal energy. These materials, also known as xe2x80x9cdry silverxe2x80x9d materials, generally comprise a support having coated thereon: (a) a photo-catalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that is capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In such materials, the photosensitive catalyst is generally a photographic type photosensitive silver halide that is considered to be in catalytic proximity to the non-photosensitive source of reducible silver ions. Catalytic proximity requires intimate physical association of these two components either prior to or during the thermal image development process so that when silver atoms, (Ag0)n, also known as silver specks, clusters, nuclei, or latent image, are generated by irradiation or light exposure of the photosensitive silver halide, those silver atoms are able to catalyze the reduction of the reducible silver ions within a catalytic sphere of influence around the silver atoms [D. H. Klosterboer, Imaging Processes and Materials, (Neblette""s Eighth Edition), J. Sturge, V. Walworth, and A. Shepp, Eds., Van Nostrand-Reinhold, New York, 1989, Chapter 9, pp. 279-291]. It has long been understood that silver atoms act as a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed in catalytic proximity with the non-photosensitive source of reducible silver ions in a number of different ways (see, for example, Research Disclosure, June 1978, item 17029). Other photosensitive materials, such as titanium dioxide, cadmium sulfide, and zinc oxide, have also been reported to be useful in place of silver halide as the photocatalyst in photothermographic materials [see for example, Shepard, J. Appl. Photog. Eng. 1982, 8(5), 210-212, Shigeo et al., Nippon Kagaku Kaishi, 1994, 11, 992-997, and FR 2,254,047 (Robillard)].
The photosensitive silver halide may be made xe2x80x9cin-situxe2x80x9d, for example, by mixing an organic or inorganic halide-containing source with a source of reducible silver ions to achieve partial metathesis and thus causing the in-situ formation of silver halide (AgX) grains throughout the silver source [see, for example, U.S. Pat. No. 3,457,075 (Morgan et al.)]. In addition, photosensitive silver halides and sources of reducible silver ions can be coprecipitated [see Yu. E. Usanov et al., J. Imag. Sci. Tech. 1996, 40, 104]. Alternatively, a portion of the reducible silver ions can be completely converted to silver halide, and that portion can be added back to the source of reducible silver ions (see Yu. E. Usanov et al., International Conference on Imaging Science, Sep. 7-11, 1998)
The silver halide may also be xe2x80x9cpreformedxe2x80x9d and prepared by an xe2x80x9cex-situxe2x80x9d process whereby the silver halide (AgX) grains are prepared and grown separately. With this technique, one has the possibility of controlling the grain size, grain size distribution, dopant levels, and composition much more precisely, so that one can impart more specific properties to both the silver halide grains and the photothermographic material. The preformed silver halide grains may be introduced prior to, and be present during, the formation of the source of reducible silver ions. Co-precipitation of the silver halide and the source of reducible silver ions provides a more intimate mixture of the two materials [see for example, U.S. Pat. No. 3,839,049 (Simons)]. Alternatively, the preformed silver halide grains may be added to and physically mixed with the source of reducible silver ions.
The non-photosensitive source of reducible silver ions is a material that contains reducible silver ions. Typically, the preferred non-photosensitive source of reducible silver ions is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, or mixtures of such salts. Such acids are also known as xe2x80x9cfatty acidsxe2x80x9d or xe2x80x9cfatty carboxylic acidsxe2x80x9d. Silver salts of other organic acids or other organic compounds, such as silver imidazoles, silver tetrazoles, silver benzotriazoles, silver benzotetrazoles, silver benzothiazoles and silver acetylides have also been proposed. U.S. Pat. No. 4,260,677 (Winslow et al.) discloses the use of complexes of various inorganic or organic silver salts.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image. This is accomplished by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced.
In photothermographic materials, the reducing agent for the reducible silver ions, often referred to as a xe2x80x9cdeveloperxe2x80x9d, may be any compound that, in the presence of the latent image, can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature sufficient to cause the reaction. A wide variety of classes of compounds have been disclosed in the literature that function as developers for photothermographic materials. At elevated temperatures, the reducible silver ions are reduced by the reducing agent. In photothermographic materials, upon heating, this reaction occurs preferentially in the regions surrounding the latent image. This reaction produces a negative image of metallic silver having a color that ranges from yellow to deep black depending upon the presence of toning agents and other components in the imaging layer(s).
The imaging arts have long recognized that the field of photo-thermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
As noted above, in photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50xc2x0 C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30xc2x0 C. to 50xc2x0 C.) to provide a visible image.
Because development is carried out with heat, the design of photothermographic materials requires that both room temperature and elevated temperature properties as well as the distribution of constituents within the material be taken into account.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example, a silver carboxylate) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the xe2x80x9cchemistryxe2x80x9d for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called xe2x80x9cinstant photographyxe2x80x9d, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of xe2x80x9cfogxe2x80x9d or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems during the preparation of the photothermographic emulsion as well as during coating, use, storage, and post-processing handling.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is, in the aqueous fixing step).
In photothermographic materials, the binder is capable of wide variation and a number of binders (both hydrophilic and hydrophobic) are useful. In contrast, conventional photographic materials are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette""s Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al., Eds., The Focal Press, London and New York, 1978, pp. 74-75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94-103, and in M. R. V Sahyun, J. Imaging Sci. Technol., 1998, 42, 23.
As noted above, thermographic and photothermographic materials generally include a source of reducible silver ions for thermal development. The most common sources of reducible silver ions are the silver fatty acid carboxylates. Other components in such materials include a reducing agent system that includes at least one reducing agent along with optional co-developers and contrast enhancing agents, and optional toning agents (common ones being phthalazine, phthalazinone, and derivatives thereof) in one or more binders (usually hydrophobic binders). These components are generally formulated for coating using polar organic solvents.
During thermal development various by-products and film components are released from the photothermographic material. These byproducts and film components, including various fatty carboxylic acids (such as behenic acid), reducing agent(s), and toners, can diffuse out of the material and build-up within the thermal processing equipment (such as on processor platens, rollers, and drums). It is important to prevent the build-up of such debris within the processing equipment since it may result in the processed material sticking to the various mechanisms and causing machine jams and scratches on the surface of the developed materials. These by-products can also build up in the imaging section of the machine and lead to image artifacts, thus impairing the quality of the image. This can be particularly critical in films designed for medical imaging applications. Service calls and machine downtime result.
During transport through imaging and processing equipment, thermographic and photothermographic coatings can be damaged by contact with mechanisms such as rollers, guides, and diverter bars within the processor. One such type of damage occurs when edges of the material rub against solid surfaces within the processor. Tiny pieces of the emulsion or topcoat may be removed and collect in various places within the processor. This debris can also cause machine jams and can build up in the imaging section of the machine and lead to image artifacts. Again, service calls and machine downtime result.
Scratches are another defect seen on imaging films. The rubbing of unimaged film on previously deposited debris or on mechanical parts within an imager is one mode of creating scratches on the surface of the imaging material. In addition, scratches can be generated during the manufacture of the photothermographic film, during roll-up or during sheeting and packaging operations.
Another type of damage, referred to as xe2x80x9cedge peel-backxe2x80x9d occurs when the leading edge of the developed material contacts diverter bars or guides directly after processing. The film, which is still hot from the preceding processing step, displays different physical characteristics from those at room temperature. The interaction between the diverter bar and the developed material can result in the topcoat layer being xe2x80x9cpeeled backxe2x80x9d from the support. At the same time, the emulsion layer is also pushed back from the support. This damages the leading edge of the material and is objectionable to the user. Edge peel-back also contributes to build-up of debris within the machine.
U.S. Pat. No. 5,422,234 (Bauer et al.) and U.S. Pat. No. 5,989,796 (Moon) describe the use of a surface overcoat layer in photothermographic materials to reduce the emission problems noted above. This overcoat layer comprises gelatin, poly(vinyl alcohol), poly(silicic acid) or combinations of such hydrophilic materials. While these overcoat layer materials provide suitable barriers to diffusion of reagents from the materials, they are typically coated from water. Coating a separate hydrophilic layer from water when the imaging layer(s) are generally coated from polar organic solvents is not desirable for a number of reasons.
While polyacrylates and cellulosic materials can also be used as barrier layer materials to provide physical protection, they do not adequately prohibit diffusion of all by-products of thermal development out of the thermographic and photothermographic materials.
Polymeric barrier layers to reduce emissions from thermographic and photothermographic films are described in U.S. Pat. No. 6,352,819 (Kenney et al.), U.S. Pat. No. 6,352,820 (Bauer et al.), and U.S. Ser. No. 09/916,366 (filed Jul. 27, 2001 by Bauer, Horch, Miller, Teegarden, Hunt, and Sakizadeh), all incorporated herein by reference. These barrier layers are all formed during coating of the heat-developable material and are either within the material or part of a topcoat layer. The barrier layers described therein do not provide any discussion of xe2x80x9cedge peel-back.xe2x80x9d
U.S. Pat. No. 3,997,346 (Masuda et al.) teaches lamination of a photothermographic film to improve the print stability of the film. The film is laminated during or after imaging and thermal development with a polymer film layer that includes stabilizer compounds. A machine that incorporates this approach will, of necessity, have a source of imaging film and a stabilizing film, with attendant mechanisms for registration, edge trimming, etc. The approach described in this patent does not address the issue of reducing scratches that occur prior to the film processing stage. Furthermore, if as described in the patent, lamination is done after processing, it will not help address the problem of emissions from the photothermographic material during imaging and development.
U.S. Pat. No. 6,124,236 (Mitchell, Jr.) teaches a direct thermal printable media wherein a thermosensitive imaging layer is applied a back surface of a thin, optically transmissive film. A supporting substrate is thereafter affixed with an adhesive to the thermosensitive imaging layer to create a laminated structure with the thermosensitive imaging layer residing between the film and the supporting substrate.
There remains a need for photothermographic materials having suitable barrier layers that provide physical protection while further reducing the emission of various by-products and ingredients.
It is therefore an object of the present invention to provide a laminated photothermographic film that captures chemical emissions during the processing step when the film is transported over a heated drum.
It is a further object of the present invention to provide a laminated photothermographic film that has improved front-side scratch resistance.
Yet another object of the present invention is to provide a laminated photothermographic film that generally eliminates peel-back when developed.
It is a further object of the present invention to provide a laminated photothermographic film that substantially eliminates debris-related defects in radiographic film where the debris is generated due to abrasion of the coated film edge.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims, and drawings set forth herein. These features, objects and advantages are accomplished by providing a photothermographic film with a complement film or web laminated thereon. The resulting laminated photothermographic film will include a support (preferably polyester). The support may include backside coating(s) of an antihalation dye, a transport matte agent and anti-static agents. The laminated photothermographic film will also include a photothermographic imaging layer. The photothermographic imaging layer is a silver-based layer that is coated on top of the support, either directly or with a carrier layer. In some situations, it may be advantageous to have a protective topcoat layer covering the silver-based photothermographic imaging layer. The complement film or web which is a pre-coated or uncoated support [such as, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polypropylene, or polycarbonate)] is laminated to the uppermost surface of the photothermographic layer that is coated on top of the support. The laminated photothermographic film may further include a tie layer and/or an adhesive layer. The tie layer, if present, is a layer coated on top of the photothermographic imaging layer (or on top of the protective topcoat layer), either separately or simultaneously with the silver/topcoat. In such cases, the complement web is laminated the uppermost layer. The adhesive layer, if present, is preferably coated on the side of the complement film that is to be interfaced with the photothermographic imaging layer or to the tie layer for the purposes of adhesion therebetween. The combination of the adhesive layer and the complement film or web are sometimes collectively referred to herein as the overlam, or overlaminate. In addition, other functional layers designed to meet one or more needs, may be coated onto the complement film or web, on the side opposite to the adhesive layer. Such layers include gloss control layers, scratch resistant layers, image receptor layers, dye receptor layers, ink receptive layers, or release layers. It will be apparent to those skilled in the art that many such and other functional properties can be built into this layer(s).
Various coating methods may be employed to coat the various layers described herein, such as extrusion coating, forward and reverse roll coating, gravure coating, slide coating, and curtain coating. The choice of the coating method is dependent in large part on the type of solution being coated, the desired thickness, and the Theological properties of the coating solutions. Many such techniques are described in E. D. Cohen and E. B. Gutoff, Modern Coating and Drying Technology, VCH, New York, 1992, and in Coating and Drying Defects: Troubleshooting Operating Problems, E. B. Gutoff and E. D. Cohen, John Wiley and Sons, New York, 1995. It will be apparent to those skilled in the art to evaluate the various options available and pick the appropriate coating technique. It must be noted that the present invention is not tied to any one specific coating technique.
The overall thickness of the laminated photothermographic film is targeted to match that of commercial medical imaging films, which have an approximate thickness of 8 mil (203.2 em). The functional layers of these films are generally coated on a support that is 6.8 to 7 mil (172.7 to 177.8 xcexcm) thick. A preferred method of preparing the laminated films of this invention is to laminate a 0.5-1.0 mil (12.7-25.4 xcexcm) film of PET over a photothermographic film, to afford a laminated construction of between 8.5 to 9 mil (215.9 to 228.6 xcexcm). It should be noted that this total thickness can also be achieved by using other combinations of complement web or film plus support thicknesses. For example, a support having a thickness of 6.5 mil (165.1 xcexcm) may be used in combination with a complement web or film having a thickness of 0.5 mil (12.7 xcexcm). In contrast, a support having a thickness of 3.5 mil (88.9 xcexcm) may be used in combination with a complement film having a thickness of 3.5 mil (88.9 xcexcm). In the practice of the present invention, for purposes of generating a desired stiffness of the film and resistance to edge peel back, the thickness of the complement film should be greater than 0.39 mil (10 xcexcm) and preferably greater than 0.5 mil (12.5 xcexcm). A maximum thickness for the complement film for a commercial medical imaging film product would be about 3.5 mil (85.75 xcexcm). Those skilled in the art will recognize that any other combination of thicknesses to achieve the desired total thickness can also be chosen based on availability, cost and other criteria.
Once the photothermographic film has been exposed to generate a desired latent image thereon thermal energy is applied to the photothermographic film to develop the latent image. Substantially all of the volatile materials present in the photothermographic imaging layer generated during the step of applying thermal energy thereto are retained between the support and the complement film.